The death of the last WW II Navajo code talker marks the end of an era for the US. This small unit of men developed an unbreakable code and was a unique operation. The skills required to do this would not be called into use today and so therefore the era cannot ever be recaptured.
Yes, the Navajo code talkers were an important part of American history and the death of the last one marks the end of an era. Code talking is a curiousity in this digital age, but there was a time when code talkers performed an important service. The death of the last one ends this chapter of history.
While the Navajo code talkers played a major part in WWII, I do not consider them central to the US armed efforts in WWII. The Navajo code talkers made up only a smart part of the veterans of WWII, and many WWII veterans are still alive. Beyond this, there were two other well established forms of code talking that were just as effective in passing coded messages during WWII, so to designate the Navajo code talkers as somehow more important and era defining than other code talkers seems disingenuous.
While it may be the death of the last code talker, it cannot be defined as an end of an era for the United States. In an era, there often is a hallmark setting that is related to a central theme that permeates a society or a generation of peoples. Thus when compared to other eras, such as the Victorian era, or Classical Era, it is arbitrary to state that the code talkers permeated the society at the time. It makes for a nice story however.